1. Field of the Invention
This invention relates to a shape memory alloy (SMA) suitable for actuators and a method of treating the same.
2. Related Art
Heretofore, upon treating a raw shape memory alloy so as to make it suitable for use in actuators, generally it has not been done to refine crystal grains and control crystal orientations of the raw shape memory alloy.
On the other hand, in order to use a shape memory alloy, it is necessary to impart a required shape to the shape memory alloy, and therefor to perform a heat treatment peculiar to each kind of shape memory alloy. This heat treatment is called xe2x80x9cshape memory treatmentxe2x80x9d and it is necessary to strictly control various conditions thereof, as it is a very delicate treatment. For example, the following methods have been well known as shape memory treatments for common Tixe2x80x94Ni based shape memory alloys. The first method, which is referred as xe2x80x9cmedium temperature treatmentxe2x80x9d, is the one wherein a shape memory alloy is sufficiently work hardened and then cold worked into a desired shape, and thereafter, held at a temperature of 400 to 500xc2x0 C. for a few minutes to several hours with the desired shape being restrained. The second method, which is referred as xe2x80x9clow temperature treatmentxe2x80x9d, is the one wherein a shape memory alloy is held at a temperature of 800xc2x0 C. or above for some time, thereafter rapidly cooled and cold worked into a desired shape, and then held at a low temperature of 200 to 300xc2x0 C. with the desired shape being restrained (xe2x80x9cIllustrated idea collection of applications of shape memory alloys in the latest patentsxe2x80x9d, written and edited by Shoji Ishikawa, Sadao Kinashi and Manabu Miwa, published by Kogyo-chousa-kai, pp. 30).
In general, conventional shape memory alloys suffer from the following shortcomings when used in actuators.
(a) The response characteristic (speed) is inferior.
(b) Usable temperature range is restricted, since Ms and Mf points (Ms being the temperature at which the martensite phase transformation starts and Mf being the temperature at which the martensite phase transformation ends) are difficult to be raised.
(c) Only a small force can be effectively extracted from the shape memory alloy.
(d) The service life before being broken is short.
(e) The shape memory alloy tends to lose the memory of an imparted configuration and permanent strain tends to be produced in the shape memory alloy for a short period of time.
(f) The strain which can be extracted from the shape memory alloy as a movement (hereinafter referred as operational strain) is decreased for a short period of time.
(g) Shape memory alloy materials, such as Tixe2x80x94Ni based or Tixe2x80x94Nixe2x80x94Cu based alloys and the like, which are intermetallic compounds having strong covalent bonding characteristic and are difficult to work, are difficult to use when they are in certain compositions, since they are very brittle and fragile.
With such shortcomings, 80 to 90% or more of applications of shape memory alloys have been those wherein they are used as superelastic spring materials and only the rest has been directed to actuators. Moreover, most of the shape memory alloys for use in actuators have been formed into the shape of a coil spring, wire or plate and have been expected to be reverted from a configuration deformed by bending or twisting and bending to the original configuration upon application of heat (in case the shape memory alloy is formed into a coil spring shape, though macroscopically or apparently it is deformed as if it were elongated or compressed upon application of a force thereto, in a true sense the deformation it is subject to is a twisting and bending one). The reason for utilizing reversion from a bending deformation or twisting and bending deformation as stated above has been that the shape memory alloy should be used so that its small strains may be multiplied since the range of its shape memory effect (SME) stably available is very narrow. Though it is said that, in conventional shape memory alloys, the maximum operational strain reaches a few percent to about 10 percent, this is true only when deformation and shape recovery are performed only once or a few times. Practically speaking, when deformation and shape recovery are repeated over large cycle numbers with regard to the conventional shape memory alloy, the operational strain is decreased and the alloy loses the memory of the imparted configuration and eventually is broken.
All of the conventional shape memory treatments intend to keep the shape stability while obtaining the pseudoelasticity and shape memory effect by partly producing microstructures which can cause pseudoelasticity and shape memory effect in microstructures of the shape memory alloy strengthened by work hardening. In other words all of the conventional shape memory treatments are those which obliges to sacrifice pseudoelasticity and shape memory effect to some extent to keep shape stability.
On the other hand, the present inventor has disclosed in U.S. Pat. No. 4,919,177 a method of treating Tixe2x80x94Ni based shape memory alloy wherein a Tixe2x80x94Ni based polycrystalline shape memory alloy material is subjected to a heat cycle which rises and drops over the transformation region of the shape memory alloy as well as to a directional energy field. According to this method, the crystal orientations of the shape memory alloy are rearranged along a specific direction and the disadvantages of the conventional shape memory alloy are overcome considerably.
However, in the method disclosed by the present inventor, the crystal grains of the shape memory alloy are not refined but caused to grow in size. Besides, since a tensile force is applied to the shape memory alloy in the final step of arranging the crystal orientations, there is a tendency that the microstructure of the shape memory alloy finally obtained is destroyed by the tensile force. Therefore, it is still not enough in overcoming the disadvantages of the conventional shape memory alloy.
It is accordingly an object of the present invention to provide a shape memory alloy having a good response characteristic and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is another object of the present invention to provide a shape memory alloy which can be used over a wide range of temperature and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is still another object of the present invention to provide a shape memory alloy from which a greater force can be practically and effectively extracted and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is a further object of the present invention to provide a shape memory alloy from which great operational strains can be extracted over large cycle numbers and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is a still further object of the present invention to provide a shape memory alloy exhibiting a huge two-way shape memory effect (reversible shape memory effect) and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is another object of the present invention to provide a shape memory alloy having a long service life and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is still another object of the present invention to provide a shape memory alloy which does not lose its memorized shape easily and a met hod of treating a shape memory alloy for obtaining such a shape memory alloy.
It is a further object of the present invention to provide a shape memory alloy of which operational strain diminishes less even with an increase of a deformation-recovery cycle number and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is a still further object of the present invention to provide a shape memory alloy which exhibits stably the aforesaid various excellent properties over large cycle numbers for a long period of time and a method of treating a shape memory alloy for obtaining such a shape memory alloy.
It is another object of the present invention to provide a method of treating a shape memory alloy which makes it possible to employ, as raw materials, those shape memory materials which have been regarded as difficult to use because of their brittleness and easiness to crack and convert them into ductile shape memory alloys in the shape of a wire or sheet etc.
It is yet another object of the present invention to provide a method of treating a shape memory alloy which makes it possible to arrange crystal orientations of a shape memory alloy without damaging the microstructure of the alloy.
Crystal grains of a shape memory alloy have orientations and there exist a plurality of orientations along which reversible slips or shearing deformations (variants), wherein microscopically relative moving ranges between the atoms of the alloy are restricted, can appear, though they are limited in number. For example, in case of a Tixe2x80x94Ni based shape memory alloy, there are as much as twenty four (24) orientations along which the deformations referred to as variants can occur. In the present invention, the crystal orientations of the shape memory alloy are arranged substantially along a direction suitable for an expected operational direction, in other words, a direction suitable for a movement of the shape memory alloy in the expected operational direction. The term xe2x80x9cexpected operational directionxe2x80x9d as herein used means a direction such as a tensile or twisting direction or the like in which the shape memory alloy is expected to move when used in an actuator after the completion of the treatment. For example, when a shape memory alloy in the wire shape is used in a contraction-relaxation fashion, the expected operational direction is a tensile direction, while when a shape memory alloy in the coil spring shape is used, the expected operational direction is a torsion direction. (In case a shape memory alloy in the coil spring shape is used, it performs shape recovery from a twisting and bending deformation upon heating. Therefore, strictly speaking, it may be said that the expected operational direction in this case is a torsion and bending direction. However, the substantial expected operational direction is a torsion direction, because bending deformation comprises a negligible percentage.).
A method of treating a shape memory alloy in accordance with the present invention comprising the steps of:
providing a raw shape memory alloy with a substantially uniformly fine-grained crystal structure; and
arranging crystal orientations of the raw shape memory alloy substantially along a direction suitable for an expected operational direction.
It is preferred that the average grain size of the raw shape memory alloy is selected to be 10 microns or less in the step of providing the raw shape memory alloy with a substantially uniformly fine-grained crystal structure. Most preferred is the average grain size in the range of 1 micron to several microns or less. With such grain size, the shape memory alloy after the completion of the treatment is particularly stable when subjected to deformation-recovery cycle.
In general, specific characteristic properties of crystalline materials are based on the phenomena in crystal grains of the materials. Accordingly, in many cases, these specific characteristic properties should naturally be most remarkably recognized when the materials are of single crystal. For this reason, when the excellent properties or functions of some material are to be utilized, in general, the best results can be obtained when the material is of single crystal. Basically the shape memory alloy is no exception on this matter. A shape memory alloy of single crystal can be deformed in a slip direction by very small force in the range where reversible slip deformation can occur under a low temperature at which it is in the martensitic phase as a whole (xe2x80x9cSlip deformationxe2x80x9d in this specification means shearing deformation which is the cause of the shape memory effect and wherein reversible movement is possible within a limited range, but it does not mean permanent and continuous slip between atoms which is the cause of the plastic deformation).
However, in practice it is extremely difficult to industrially produce the single crystalline material, and the production of it, even when achieved, should be very expensive. Besides, in case of a shape memory alloy, when it is of single crystal, its microstructure becomes unstable.
Of course conventional shape memory alloys are polycrystalline substances, and in general, orientations of the respective crystals thereof have been random and the grain sizes of respective crystals are uneven, and thereby it is thought that aforesaid various shortcomings are caused (this will be discussed later in detail).
The present inventor has found that a shape memory alloy can be obtained which has both advantages of the single crystalline shape memory alloy and those of the conventional polycrystal shape memory alloys, when the shape memory alloy, as in the present invention, is formed of a polycrystal material and provided with a substantially uniformly fine-grained crystal structure, and the crystal orientations thereof are arranged along a direction suitable for an expected operational direction. When the crystal grain sizes of the alloy are made substantially uniform and the crystal orientations are arranged along a direction suited for a desired movement of the alloy, even if gigantic shape recovery force is produced in respective crystal grains, no part of the alloy is subject to an excessive deformation and the internal structure of the alloy is difficult to destroy. Besides, when respective crystal grains are adequately small, structural contradictions caused by differences between deformation directions of the respective crystal grains, etc. are also small and thereby the respective crystal grains themselves are difficult to destroy. Moreover, in such a material, since the volume proportion of the structure at and around the crystal grain boundaries to that within the grains is comparatively larger, its ability to absorb the structural contradictions is high. Further, such a material can be reformed into a shape memory alloy in the shape of a wire or sheet etc. which is sufficiently ductile over a wide strain range, even in the case where it is brittle when it is a raw material. The reason for this is presumed that, in such a material, the structure at and around the crystal grain boundaries exhibits properties like those of an amorphous material. Even the respective crystal grains are fine, if the crystal orientations are arranged, comparatively large shape memory effect can be extracted from the shape memory alloy. A force required to deform the shape alloy is small, since the orientations of the respective crystals along which the crystals are easy to move are arranged in the same direction. Because the volume proportion of the structure at and around the crystal grain boundaries to that within the grains is comparatively larger, large elastic energy can be stored at and around the crystal grain boundaries without employing the measures of depositing impurities there, or the like, and thereby a stable and large two-way shape memory effect can be obtained as well as the property that a force required to deform the alloy is small.
Thus, the shape memory alloy in accordance with the present invention has the following excellent properties, though some of them have been already mentioned above.
(A) Since the temperature hysteresis is small on the temperature-stress diagram and the transformation temperature range is narrow, heating and cooling of the alloy can be taken place quickly, the response of the alloy is good, and a high-speed reciprocating motion can be achieved. For example, when applied to a Tixe2x80x94Nixe2x80x94Cu based shape memory alloy, the temperature hysteresis can be almost zero over a comparatively wide range. A successive reciprocating operational strain reaching to almost 80% of that in the full stroke (strain xcex5=4%) could be successfully extracted from a shape memory alloy in accordance with the present invention with a 150 Mpa load and a temperature difference of merely 10xc2x0 C. This, when compared to a engine, is equivalent to the revolution speed is remarkably raised with the same size. Accordingly, it is equivalent to that the horsepower as well as the load capacity is considerably raised. A significant improvement of the responsiveness can be expected, when used in a mechanism such as a servo actuator wherein two-way movement is required.
(B) The force which can be practically extracted from the shape memory alloy (hereinafter referred as recovery force) can be increased. The recovery force does not depend on the maximum recovery stress but the limit of the stress repeatedly usable in consideration of fatigue of the alloy, etc. When compared to a engine or motor, the recovery force corresponds to the maximum torque. With the shape memory alloy treated by the method in accordance with the present invention, the limit of the stress practically available in the reiterative operation is high, even when the maximum recovery stress is same as that of the conventional shape memory alloy. The conventional shape memory alloy has a small recovery force, and if operated repeatedly with an excessively large stress applied thereto, it suffers from loss of the memory of the imparted configuration, decrease of the operational strain and rupture, as stated above. It means shortening of service life of the actuator. This is the reason why most of conventional shape memory alloy actuators have been formed in the shape of a coil spring, as previously stated. With the coil spring shape, the strain produced in the alloy is very small when the alloy is deformed. Therefore, the stress actually used has been considerably small as compared with the maximum stress practically available.
(C) Large operational strains can be extracted over large cycle numbers. The shape memory alloy in accordance with the present invention, when formed into a rectilinear shape, can achieve a deformation-shape recovery cycle with a tensile strain of 5% or more. The value of the operational strain, 5% or more stands comparison with that a 1 m long round bar is expanded and contracted by 5 cm or more. This magnitude of strain is much larger than that of the strain which an ordinary coil spring is subject to when it is deformed and restored between the coil and rectilinear shape. This value is much larger than the ranges of strains available in case of conventional shape memory alloys including superelastic alloys. When the treatment in accordance with the present invention is applied to a brittle raw shape memory alloy material such as Tixe2x80x94Nixe2x80x94Cu based shape memory alloy and the like, huge operational strains as stated above can be extracted stably over more than one hundred million cycles. When the conventional shape memory alloy is used in the coil spring shape, in most cases, the moving strain is less than 0.1% in tensile strain equivalent. In other words, in most cases, the coil spring of a shape memory alloy has been used with almost the same magnitude of displacement as the coil spring of a non-shape memory alloy metal such as iron and the like.
(D) It is possible to cause a shape memory alloy to exhibit a huge two-way shape memory alloy effect. The two-way shape memory effect is a phenomenon wherein a shape memory alloy recovers the original configuration upon heating and deforms into another configuration upon cooling, and no force or only a very small force is required when the alloy is subject to the deformation at a low temperature in a direction opposite to the shape recovery. Apparently, it appears that the shape memory alloy remembers two configurations, the deformed configuration at a low temperature and the original configuration at a high temperature. For instance, in case the shape memory alloy is rectilinear and the deformed configuration (length) thereof is the one stretched from the original configuration (length), the shape memory alloy contracts to the original length and becomes hard upon heating, while it extends by itself to the deformed length and becomes soft just like a muscle relaxes upon cooling, even in the absence of a load. In other words, the shape memory alloy expands and contracts, driven by heating and cooling alone in the absence of a bias force from the outside. According to literature, etc., it has been thought that, generally the two-way shape memory effect is a phenomenon observed only within the range wherein a strain xcex5 is 1% or less in tensile strain equivalent and it is difficult to put it to practical use since it is unstable. In fact, hitherto devices utilizing the two-way shape memory effect have been hardly found.
According to the present invention, however, it is possible to cause a huge two-way shape memory effect almost over the whole range wherein the shape memory effect occurs, namely, the whole range of recoverable strain. According to the present invention, the two-way shape memory effect with a strain of 5% can be exhibited even in the absence of a load. The present inventor postulates that, since the polycrystal shape memory alloy in accordance with the present invention has crystals each of which orientation, size and position are adapted to deformations from the outside, a stable two-way shape memory effect can be induced almost in the whole range of the operational strain, if there exists in the alloy the slightest level of a residual stress field resulted from the working in a direction opposite to the shape recovery direction. This huge two-way shape memory effect appears stably over about one hundred million cycles in the absence of a load.
(E) The shape memory alloy in accordance with the present invention has a long service life. The conventional shape memory alloy has a service life of about one hundred thousand cycles, at the largest, even with the small operational strain. Particularly, in case a movement wherein the operational strain exceeds 2% in tensile strain equivalent is performed, there is a tendency that its service life becomes extremely short. However, the shape memory alloy in accordance with the present invention provides a stable movement over one hundred million cycles with a huge operational strain reaching nearly 5%.
(F) The memory of the imparted configuration and the range of the operational strain are stable, that is, the memory of the imparted configuration and the range of the operational strain do not diminish with cycle number of the deformation and recovery or do only slightly. In other words, the magnitude of the operational strain has little effect on the service life of the shape memory alloy. The reason for it is postulated that the shape memory alloy in accordance with the present invention has the orientations, sizes and arrangements of the respective crystals in a state adapted to deformations from the outside. It is presumed that the deformation from the outside is undertaken, to a certain extent, mainly by the crystals which achieve a huge reversible thermo-elastic deformation that is characteristic of a shape memory alloy, while the deformation larger than it is undertaken by the structure at and around the crystal grain boundaries wherein a reversible thermo-elastic deformation is hardly produced. With such structure of the shape memory alloy, displacements, plastic deformations and rotations of the respective crystal grains are hard to occur even with large cycle numbers, and the alloy is hardly subject to a permanent deformation.
(G) Even when the raw material is brittle, it can be reformed into a ductile shape memory alloy in the shape of a wire, sheet or the like. The shape memory alloy in accordance with the present invention has higher apparent ductility than shape memory alloys treated by the conventional shape memory treatment since it consists of the fine crystal grains reversibly deformable and the structure at and around the crystal grain boundaries which exhibits amorphous like properties and occupies a considerable part of the alloy with regard to volume.
(H) The various excellent properties of the shape memory alloy mentioned above are stable over a long time of period and large cycle numbers.
In a particular aspect of the method of treating a shape memory alloy in accordance with the present invention, the step of providing a raw shape memory alloy having a substantially uniformly fine-grained crystal structure comprises the steps of:
heating the raw shape memory alloy in an amorphous state or a state similar thereto to the temperature at which recrystallization begins or a little above for a short period of time, with a stress applied to the raw shape memory alloy in the expected operational direction at least in the stage where a recovery recrystallization begins, to produce a substantially uniform fine-grained crystal structure with an anisotropy in the expected operational direction, while relaxing the internal stress generated in the raw shape memory alloy in the expected operational direction.
In case the raw shape memory alloy is not in an amorphous state or a state similar thereto, the raw shape memory alloy can be be put into a state similar to amorphous state, for instance, by being subject to a severe cold working. It is preferable that the severe cold working is achieved at a cryogenic temperature which is sufficiently lower than the temperature singular point B of the raw shape memory alloy. The point B is an inflection point observed in the sub-zero temperature range and is associated with transitions of the physical property values such as specific heat, electrical resistance and the like (This will be explained later in more detail). The object for this is to completely transform nonmartensite structures remaining in the alloy, even if the amount of them are very small, into the martensite. In general, the so called martensite finished point Mf at which the shape memory alloy transforms completely from austenite to martensite is the temperature which is measured with respect to a specimen completely annealed. In worked materials, however, there remain a considerable amount of the non-martensite structures even at this temperature. The non-martensite structures may be retained austenite, a structure resulted from work hardening or the like.
Upon heating the raw shape memory alloy to the temperature at which recrystallization begins or a little above for a short period of time, the raw shape memory alloy may be either in a state where a stress is applied to it in the expected operational direction or where it is constrained in a shape not loosened in the absence of a load. At this stage, since the raw shape memory alloy has a martensitic component which can recover the shape in the expected operational direction upon heating, if it is constrained in a shape not loosened in the absence of a load, a stress is produced in the expected operational direction while heating and thereby the same result is obtained as when the alloy is constrained with a stress applied thereto prior to heating as stated above. What is essential is that at least when a recovery recrystallization begins the raw shape memory alloy is in a state where a stress is loaded thereto in the expected operational direction.
In the particular aspect of the method of treating a shape memory alloy in accordance with the present invention, the step of arranging crystal orientations of the raw shape memory alloy comprises the steps of:
subjecting the raw shape memory alloy to a high level of deformation by means of a stress in the expected operational direction at a very low temperature at which the austenite phase does not remain in the raw shape memory alloy so that a slide deformation is introduced into the crystal grains of the raw shape memory alloy which have been transformed completely into the martensite phase, within a reversible range along the direction of the stress;
heating the raw shape memory alloy to a temperature between Af (a temperature at which the austenitic transformation ends) and the recrystallization temperature with a stress applied to said raw shape memory alloy in the expected operational direction so that the directions of reversible slip motions of the respective crystal grains of said raw shape memory alloy are arranged in the expected operational direction.
The crystal orientations of the raw shape memory alloy are arranged when the directions of reversible slip motions of the respective crystal grains are arranged in the expected operational direction. Hereupon, the orientation of crystal grain means the one where a reversible slip deformation due to the martensitic transformation is easy to occur practically such as one of orientations of variants and the like, but not necessarily one and the same orientation from the view point of the crystallography.
The step of introducing a slide deformation to the crystal grains and that of arranging the directions of reversible slip motions of the s crystal grains may be repeated a required number of times. Generally it suffices to repeat one to three times.
In the method of treating a shape memory alloy in accordance with the present invention, it is preferable to take place a step of running-in, after having rearranged the crystal grains of the raw shape memory alloy along the direction which is suited for the reversible deformation of the alloy in the expected operational direction as stated above, in order to obviate instability of the alloy which appears in the initial stage of its repetition movement. This running-in step is a process which aims for the same effect as the xe2x80x9ctrainingxe2x80x9d process which has been employed in the conventional shape memory treatment.
Preferably, the running-in step is performed, after arranging the directions of reversible slip motions of the respective crystal grains of the raw shape memory alloy in the expected operational direction, by subjecting the raw shape memory alloy to a heat cycle between a temperature of Mf point or below and a temperature at which only a high level of plastic deformation is relaxed, while controlling a stress applied to the raw shape memory alloy without restraining the strain introduced in the raw shape memory alloy. In general, it is preferable that a few to several tens cycles of the heat cycle is applied to the raw shape memory alloy. In accordance with the running-step, a work hardening and a structural defect having an elastic energy field which contribute to the dimensional stability and two-way shape memory effect of the alloy can be stored in the microstructure at and around the crystal grain boundaries to the desired degree and thereby the instability of the alloy which appears in the initial stage of its repetition movement can be dissolved.
It has not been yet fully elucidated theoretically what phenomenon occurs in the shape memory alloy and why the alloy exhibits various excellent properties as stated above when the treatment in accordance with the present invention is carried out. However, to make the present invention easily understood, a supplementary explanation will be given hereunder on the basis of a hypothesis the present inventor holds at present.
It is considered that in a polycrystalline shape memory alloy each crystal performs as a single crystal, while the structure at and around the crystal grain boundaries connects the crystals with each other. Therefore, in case orientations and sizes of the crystals are random, when the respective crystals present large deformations due to the superelasticity and shape memory effect, the structure at and around the crystal grain boundaries is subject to structural contradictions caused by the deformations of the crystals. The conventional shape memory alloy, treated with an ordinary shape memory treatment after manufactured by ordinary working such as casting, hot working and the like, is polycrystalline and random in the crystal orientations and sizes thereof, and some of the crystals thereof have been destroyed by strong working. Such circumstances constitute obstacles disturbing a smooth deformation and shape recovery of the alloy, and thereby a considerable force is required to deform the alloy, even when at a temperature sufficiently low for the martensitic transformation to be completed. Therefore, satisfactory shape memory effect can not be achieved when it is used as an actuator, even after the shape memory treatment.
The shape recovery force within the crystal grain is strong and has enough magnitude to deform plastically and destroy the structure at and around the crystal grain boundaries which constitutes a connection between crystal grains and the crystal grains which is not yet in the shape recovery state. This may explains the reason why the conventional shape memory alloy soon loses the memory of the imparted shape and becomes hard, with the operational strain thereof decreasing, when it is subject to repetitions of a large deformation and shape recovery. It may be because the interior of the shape memory alloy is changed little by little due to the great shape recovery force. Especially, in the case where the shape memory alloy performs the shape recovery when it is subject to a large deformation and restrained in the deformed configuration, the shape recovery forces of the respective crystal grains act on the interior of the alloy material at a stretch and the shape memory alloy deteriorates rapidly. The fact is that, in case of the conventional shape memory alloy, the superelastic spring and the like, the above-mentioned defect should be covered up by practicing strong working to cause work hardening in the alloy, and consequently constructing the internal structure in the alloy where the huge shape recovery forces of the crystals are restrained.
On the other hand, in accordance with the present invention, the sizes of the crystal grains being made even and the orientations thereof being arranged along the predetermined direction, even if a huge shape recovery force is produced in each crystal grain, there is no part in the alloy where an excessive deformation is produced and the internal structure of the alloy becomes hard to break. Besides, if the respective crystal grains are adequately fine, structural contradictions produced due to the differences between the orientations of the respective crystal grains or the like are small, and the crystal themselves becomes hard to break. Moreover, in such a fine-grained material, since the volume proportion of the structure at and around the crystal grain boundaries to that within the grains is comparatively larger, the ability to absorb the structural contradictions is high. Further, probably as the structure at and around the crystal grain boundaries exhibits properties like those of an amorphous material, it can be converted into a shape memory alloy in the shape of a wire or sheet, etc. which is sufficiently ductile over a wide strain range, even in the case where it is brittle as a raw material. Though the respective crystal grains are fine, since the crystal orientations are arranged along the specific direction, a comparatively large shape memory effect can be extracted from the shape memory alloy. The force required to deform the shape alloy is small, since the orientations of the respective crystals along which they are easy to move are arranged along the specific direction. Because the volume proportion of the structure at and around the crystal grain boundaries to that within the grains is comparatively larger, large elastic energy can be stored at and around the crystal grain boundaries without employing the measures of depositing impurities there, or the like, and thereby a stable and large two-way shape memory effect can be obtained as well as the property that a force required to deform the alloy is small.
When crystal orientations of a shape memory alloy are random, the larger the average grain size of the shape memory alloy is, more conspicuously the shape memory effect occurs. However, in that case, stability as a material is deteriorated. The reason for it is thought that structural contradictions are liable to be produced in the alloy due to the large grain sizes and random crystal orientations, causing changes of structure in the alloy. For instance, a treatment for a shape memory alloy generally called xe2x80x9chigh temperature treatmentxe2x80x9d has been known wherein the shape memory alloy is annealed sufficiently at a high temperature. According to this treatment, because the crystal grain sizes become larger, a large shape memory effect can be induced, but loss of the memorized shape, generation of a permanent deformation and decrease of the operational strain, etc. are caused soon with a deformation-recovery cycle number. Accordingly, though large operational strains can be extracted, the alloy becomes functionally unstable, and thereby nowadays this high temperature treatment is not put to practical use. On the contrary, when the crystal grains are fine, though the magnitude of the shape memory effect decreases relatively, the shape memory alloy becomes materially stable, since structural contradictions produced in the alloy due to the movement of the respective crystals become small and affect less the respective crystals.
Besides, as stated before, with a fine-grained structure, the volume proportion of the structure at and around the crystal grain boundaries to that within the grains is larger, as compared with in the case of a coarse-grained structure. Accordingly, the properties of the boundaries of crystal grains appears outside conspicuously. It is considered that the structure at around the crystal grain boundaries is in disorder and amorphous like properties are dominant there, as compared with the interior of the crystal grain which has a well-ordered atomic arrangement. The metal structure at and around the crystal grain boundaries and that within the grains are structurally different material, though they make little difference in composition. Naturally, the properties of the metal structure at around the crystal grain boundaries must differs very markedly from those of the metal structure within the grains. While it is easy to impart a deformation related to the shape memory effect to the structure within the crystal grains, it is difficult to impart such deformation to the structure at around the crystal grain boundaries, since it is constrained, getting between the crystal grains, and has poor reversible deformability. Therefore, it is considered that the metal structure at and around the crystal grain boundaries and that within the grains are two different materials. As a matter of course, transformation points within crystal grains differ from those at and around the crystal grain boundaries. It is thought that the process of rearranging the crystal orientations along the specific direction in the present invention uses the aforesaid properties of the crystal grain boundaries and therearound.
Most of conventional shape memory alloy production methods and shape memory treatments control strains of the shape memory alloy to define a shape of a finished shape memory alloy and a memorized shape. On the contrary, one of the distinguishing characteristics of the present invention is that most of the main processes thereof are carried out in a state where not the strain but the stress is controlled, allowing the raw shape memory alloy to deform freely. By not controlling the strain, the present invention utilizes the property of the shape memory alloy that the alloy itself reconstructs the internal structure thereof to be adapted for the movement circumstances thereof.
Besides, since the entire treatment process is carried out in rapid dynamic heating and cooling operations, long spells of heat treatment is not required unlike in the case of conventional treatments, though the procedure is comparatively complicated. Therefore, a high speed and consecutive large-scale process for treating a shape memory alloy material can be attained which provides a high-performance shape memory alloy.
Shape memory alloys, more particularly Tixe2x80x94Ni based and Tixe2x80x94Nixe2x80x94Cu based shape memory alloys are not ordinary alloys consisting of two or more metals simply mixed together but intermetallic compounds having strong covalent bonding character. Due to the strong covalent bonding character, they have characteristics like those of inorganic compounds such as ceramic and the like, though being metal. Free electrons are restrained considerably within them because of the strong covalent bonding as compared with the case with metallic bond. Smallness of the free electron movement within them is supported by their properties of poor heat conduction and high electric resistance, though they are metal. The difficulty of free electron movement makes it hard for the fusion and reorganization of the electron cloud to occur. This is a strong reason that Tixe2x80x94Ni and Tixe2x80x94Nixe2x80x94Cu based shape memory alloys are brittle materials which are hard to plastically deform. Though the treatment in accordance with the present invention can be applied to all kinds of shape memory alloys, particularly it is very effective when applied to shape memory alloys, such as Tixe2x80x94Ni or Tixe2x80x94Nixe2x80x94Cu based shape memory alloys or the like, which have strong covalent bonding character and are brittle as raw materials. When the treatment is applied to such materials, the service life, the moving range and the dimensional stability thereof are remarkably improved especially in repetition action under a heavy load, and the ductility thereof is also improved. Moreover, it becomes possible to use alloy compositions which hitherto have been considered to be no use for shape memory alloys, as alloys with them being hard to work or being too brittle even though possible to be worked. Accordingly, it can be expected to create new shape memory alloys which have unprecedented properties.
In another particular aspect of the method of treating a shape memory alloy in accordance with the present invention comprises the steps of:
subjecting a raw shape memory alloy having an anisotropy in an expected operational direction to a high level of deformation by means of a stress in the expected operational direction at a very low temperature at which the austenite phase does not remain in the raw shape memory alloy so that a slide deformation is introduced into the crystal grains of the raw shape memory alloy which have been transformed completely into the martensite phase, within a reversible range along the direction of the stress;
heating the raw shape memory alloy to a temperature between Af and the recrystallization temperature with a stress applied to said raw shape memory alloy in the expected operational direction so that the directions of reversible slip motions of the respective crystal grains of the raw shape memory alloy are arranged in the expected operational direction.
In this case, the raw shape memory alloy is not necessarily should have substantially uniformly fine-grained crystal structure. According to this aspect, also the crystal orientations are arranged along the direction suitable for the expected operational direction without breaking the structure of the shape memory alloy, as in the aforesaid aspect.